James Knott wrote:
Hans Witvliet wrote:
On Fri, 2007-12-07 at 12:59 -0500, James Knott wrote:
The Data General Eclipse line had a feature called "Writable Control
Store", which could be used to add custom instructions to the CPU. The
VAX 11/780 had it's microcode loaded from floppy at boot, but I don't
recall if it was changeable in the same manner as the Eclipse WCS.
Floppy?
It was a real huge 8" flop ... (still have them here)
hw
Yep. There was an LSI-11 (microprocessor version of PDP-11) hidden in
the cabinet, equipped with one or two 8", hard sectored drives. It was
also used to connect the console terminal. As I recall, the command to
use it as the VAX console was "STP" and Ctl-Z(?) to return to the LSI-11
console. Back in it's day, the VAX was considered a "super mini", a
real hot system. But it only had the CPU power of a 386!'
The old VAX had a beautiful assembly language with 13 different
addressing modes (4 numerically different addressing modes were
all used for a 6-bit "immediate data" addressing mode)...which,
when paired with the brilliance of making the program counter
and stack pointer general registers (and thus specifiable as
the register to reference in all of those addressing modes) had
a grand total of something like 20 effective addressing modes.
GREAT design if you were writing assembly code. Unfortunately,
all of those crazy addressing modes (like doubly-indirect or
something like that) made for a real mess whenever a page
fault occurs. Tthe CPU has to unwind the whole partially-
complete instruction, and then do an interrupt to the VM's
swapping routine, return from the swapper routine, and then
restart the instruction... And if the instruction was
something like insert a record into a doubly-linked list,
or performing some sort of string manipulation (some of
which are more complicated than what's in C's strings library),
well, then... yeah....it's amazing the VAX guys ever got
the design to function right.
Initially, i thought RISC was a crazy idea, compared to
the ultra-CISC VAX-11 instruction set...but then when I
read about the issues with page faults... I was almost
immediately convinced that yes indeed, the RISC guys did
have a very, very important point, and that the Clipper
chip and others weren't so crazy after all.
BTW, that's where I first came across the "Adventure" game. :-)
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